It is known (Strafford K. N., "High temperature Corrosion of alloys containing rare earth or refractory elements: a review . . . ," High Temperature Technology Vol. 1, No. 6, November 1983) that metallic alloys of the type MCrALX and of the type MCrALZX, where M is iron and/or cobalt and/or nickel and X represents small amounts by weight of added highly reactive elements, such as Y, Zr, Ti, Ce, Sm, Hf, La, Th, U, V, W, Ta, Nb, Mo, Gd, Si, Mg or Ca, and Z is an element, or an oxide thereof, from the series comprising X but is a different element from that chosen in each case for X, improve the properties of the oxide layer. Adhesion of the oxide layer, which consists of individual oxide particles, is improved, and the oxidation behavior is advantageously affected.
It is also known (Ramanarayan T. A., Raghavan, M. and Petkovic-Luton, R., "The Characteristics of Alumina Scales formed on Fe-Based Yttria-Dispersed Alloys", J. Electrochem. Society, April 1984, Vol. 131 No. 4., 923-931) that particularly finely dispersed oxides of the rare earths, such as Y.sub.2 O.sub.3, produce a similar improvement in a base alloy.
The alloys mentioned are known to form either layers consisting predominantly of chromium oxide or aluminum oxide, with selectivity, or layers consisting of Al.sub.2 O.sub.3 /chromium oxide mixed crystals, depending on the composition. Where the temperatures used are about 900.degree. C. or higher, the alloys which form Al.sub.2 O.sub.3 layers are chosen.
Where their use entails particularly frequent temperature changes with high temperature differences, the disadvantage of the layers and components consisting of the alloys described and produced by the process described is that scaling of individual areas of the oxide layer takes place. Although the defects thus produced are eliminated again by the stated alloys under suitable conditions, the occurrence of scaling during intensive thermocycling restricts the useful life and makes the materials known today unsuitable for some applications, for example as supports for catalytically active substances, in particular noble metals, for the detoxification of combustion gases.
Another disadvantage is that alloys containing more than about 6% of aluminum, according to U.S. Pat. No. 4,414,023 discussed below, cannot be produced in the form of foil by rolling, or the production of this foil entails substantial costs. For long-term thermal stability, however, elimination of defects in the oxide layers by additional aluminum to be supplied from the alloy is necessary, and as high an Al content as possible is also useful.
U.S. Pat. No. 4,414,023 discloses a ferritic stainless steel alloy which is hot workable and is resistant to thermal cyclic oxidation and scaling at elevated temperatures. The iron-chromium-aluminum alloy contains cerium, lanthanum and other rare earths and is suitable for forming thereon an adherent textured aluminum oxide surface.
In making this alloy, a melt is prepared in a conventional manner. Preferably, the normal steelmaking impurities of oxygen, nitrogen and sulfur are reduced prior to the addition of rare earths to the melt. Any conventional processes, including electric arc furnaces, AOD and vacuum induction melting processes, are acceptable. The melt is then cast into ingots, bars, strips or sheets. The steel is subsequently hot and/or cold rolled and subjected to conventional processes such as descaling and heating prior to fabrication into the desired shape. The ferritic stainless steel is then heat-treated to form an aluminum oxide surface.
The object of the invention is to provide a metallic semi-finished product which, preferabIy in the absence of a wash coat, constitutes a substrate for a catalytically active coating, the said substrate being stable to thermal cycling. A further object is to provide a process which leads in as simple a manner as possible to a metallic semi-finished product having the stated property.